Development effort in extreme-ultraviolet (EUV) spectral region has been stimulated by the progress of the next generation lithography systems for the semiconductor industry. The formation of highly reflective multilayer coatings on substrates is one of the key techniques enabling the next generation EUV lithography systems because there are no optical materials that are transparent in the EUV region and all optics has to be reflective. Accordingly, there is considerable worldwide interest in mirrors with high reflectivity in the EUV. Among various coating material combinations that have been tried, Mo/Si based multilayer systems exhibit particularly high theoretical reflectivity. Due to the extremely short wavelengths used in the EUV region (approximately 10-15 nm, with 13.5 nm being the wavelength of choice), the individual layer thicknesses in the Mo/Si multilayer system are only a few nanometers, each layer ranging from approximately 2 nm to approximately 5 nm. Typically a total of approximately 60 periods is required for a high reflective Mo/Si system, wherein one period corresponds to a period of single layer of Mo and a single layer of Si. For example, FIG. 1 illustrates reflectance versus wavelength for a EUV high reflector designed to operate at 13.5 nm using a Mo/Si multilayer structure. FIG. 2 is a schematic drawing of the cross-sectional view of an EUV mirror having a plurality of Mo/Si periods, for example 60 periods, on a substrate 30, the substrate with the 60 periods being designated 30 13(Mo,Si)60. The individual layer thicknesses are Mo=2.8 nm for Mo and Si=4.2 nm, respectively, and the total number of individual layers is 120 consisting of 60 periods, each period having one Mo layer and one Si layer. FIG. 3 is a graph illustrating spectral reflectance shift due to a 0.3 nm Mo layer thickness reduction (decrease from 2.8 nm to 2.5 nm) in the 60-period Mo/Si system. The 0.3 nm thickness reduction leads to 0.55 nm center wavelength blue-shift; that is, the center wavelength shifts from 13.5 nm to 12.9 nm as a result of 0.3 nm thickness change of the Mo layers. Consequently, in view of the fact that small variances in the Mo layer thickness can cause a relatively large (˜11%) shift in wavelength center), it is necessary to precisely control Mo/Si layer thicknesses when depositing the materials to form EUV mirrors so that the wavelength of the highly reflective coating remains centered at 13.5 nm. In addition, achievement of high reflectance at such a short wavelength as 13.5 nm requires a smooth surface and sharp interfaces at the sub-nanometer scale.
The challenges mentioned above in the making EUV coatings have led worldwide effort, primarily at research institutions, to find technical solutions for depositing qualified Mo/Si multilayer systems over the pass decade. Sasa Bajt et al (J. Appl. Phys., Vol. 90, No. (2) (2001), page 1017 et seq, of the Lawrence Livermore National Laboratory (LLNL) reported amorphous-to-crystalline transition in Mo/Si multilayers deposited by magnetron sputtering. Norbert Kaiser et al (SVC 51st Annual Technical Conference Proceedings, Vol. 53 (paper 20-08) of the Fraunhofer Institute and Zhanshan Wang et al of the Pohl Institute (Proceedings SPIE, Vol. 5963, (2005), paper 59630S-1) employed DC-magnetron sputtered Mo/Si multilayers for the EUV and soft X-ray optics. For larger size EUV optics, N. Benoit et al (Applied Optics, Vol. 47, No. 19 (2008), page 3455 et seq) of the Fraunhofer Institute installed a Kenotec DC-magnetron sputtering system in addition to Leybold DC-magnetron system described by Kaiser et al (op cit.) in their earlier work. As the need for EUV lithography becomes greater, the industry has started to accelerate Mo/Si multilayer development. For example, Kenji Ando et al at Canon Incorporated designed a new magnetron sputtering apparatus for EUV optics that is described in U.S. Pat. No. 7,229,532. Masayuki Shiraishi at Nikon has described the use of magnetron sputtering to make multilayer-film mirrors for lithography systems as has been described in U.S. Pat. No. 7,599, 112. While magnetron sputtering has thus been used to make optical coating for use in the EUV range optical coatings, and is the most commonly used deposition technique as shown by the work done at LLNL in the US, the Fraunhofer Institute in Germany, the Pohl Institute in China, and Nikon and Canon in Japan, there are disadvantages to the technique. Among the disadvantages to investing in a DC-magnetron sputtering system for EUV coating development are the high cost of ownership and low coating flexibility. Typical DC-magnetron sputtering systems have a multi-million dollar cost and the cost-of-ownership of magnetron sputtering system is also high; for example, the large size, high purity solid targets used in magnetron sputtering system are very expositive, the target surfaces need to be frequently refurbished, and the targets have to be frequently replaced. In contrast to magnetron sputtering, plasma ion assisted deposition (PIAD) is a well established coating technology. PIAD has been extensively used for oxide coatings (J. Wang et al in (a) Applied Optics Vol. 47, No. 13 (2008), pages C189-192; (b) Applied Optics, Vol. 46, No. 2 (2007), pages 175-179; and (c) Applied Optics, Vol. 47 No. 13 (2008) pages C131-134 and oxide-fluoride hybrid coatings. There are many technical advantages to the use of PIAD, for example, the low cost-of-ownership and high coating flexibility. In addition, it is very easy to change deposition materials, which enables one to use PIAD to support various projects simultaneously. However, it is believed that PIAD is only good for dielectric coatings such as oxide coating materials; there are technical roadblocks that restrict the PIAD for Mo/Si multilayer EUV coatings.